Spin-orbit coupling electron delocalization

Noodleman, L., C. Y. Peng, D. A. Case, and J. M. Muesca. 1995. Orbital Interactions, Electron Delocalization and Spin Coupling in Iron-Sulfur Clusters. Coordination Chemistry Reviews 144, 199. 

In the experimental UPS of HRe(CO)5 (87, 161, 174) the splitting of the first and second bands is 0.31 eV and that between the second and third bands is 0.34 eV (Fig. 9). These relative spacings are consistent only with Scheme III, thus confirming that the e MO is less stable than the b2 MO. The spin—orbit coupling parameter, f5d(Re), of 0.25 eV deduced from the spectra compares with the range 0.26-0.34 eV estimated for the Re2 cation (136, 156). The reduction of fsd(Re) in HRe(C0)4 is consistent with the delocalization of metal 5d electrons into the 2ir MOs of the CO ligands. 

The model just described, which is derived on the basis of pure crystal field theory, can be modified rather simply to include the effects of electron delocalization. Since the ab initio calculation of crystal field terms is at best totally unreliable, we follow the usual practice of treating these terms simply as adjustable parameters. Thus the question of paramount importance is how the spin-orbit coupling constant i is affected by covalent bond formation. 

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